Caught Between Male and Female

Dec. 6, 2013 7:03 p.m. ET

We mammals are obsessed with classifying individuals by sex. For humans, it's "She had her baby? Great. Boy or girl?" A baboon asks the same, ambling over to a newborn and prying its legs open to have a look. And the same goes for dogs, meeting and greeting by sniffing each other's privates: What kind are you?

People typically think of mammals as coming in two clear-cut sexual flavors. But, as the ambidextrous will attest,...